fsolve - How do I input known variables and still make a guess at the unknown variables in fsolve?

Hi,

I am working with 6 highly non-linear equations in which I define two variables (A1 and A2), input 4 variables from the previous timestep (B1 -> B4), and using the fsolve command, have a guess at 6 values, 4 of which will become the new B1->B4 values for the next timestep (C1->C4), and 2 which I am actually interested in (D1 D2).

I have my code in the form:

    function EoM = myfun(...)  % the inputs to this are numerous and not all of them will be guessed, some are in the form of arrays
                  % Define a series of coefficients and use them with the inputs to determine the equations to be used by fsolve
  EoM = [/*some function including A1,  A2, B1, B2, B3, B4, C1, C2, C3, C4, D1, D2*/;...
         /*another function including A1,  A2, B1, B2, B3, B4, C1, C2, C3, C4, D1, D2*/;...
         .    ;...
         .    ;...
         .    ;...
         /*the final equation*/];
  end

Guess = [C1 C2 C3 C4 D1 D2]
Ouput = fsolve(@myfun,Guess)

if true
    % code end

I guess my question is, will this work or will fsolve be looking for guesses at the other values?

cheers for any help

Answer 1

[Matt J]

You should use anonymous functions to tell FSOLVE which parameters are known/fixed and which are unknowns to be solved for. In other words, your code should look something like this

   function EoM = myfun(UnKnowns,Fixed)  

     tmp=num2cell(Unknowns);
     [C1,C2,C3,C4,D1,D2]= deal(Unknowns{:});

     tmp=num2cell(Fixed);
     [B1,B2,B3,B4,A1,A2] = deal(Fixed{:});

     EoM = ...

   end

Guess = [C1 C2 C3 C4 D1 D2];
Fixed= [B1,B2,B3,B4,A1,A2];

Ouput = fsolve(@(Unknowns)myfun(Unknowns,Fixed), Guess)

[Matt J]

See also other techniques for passing extra parameters

http://www.mathworks.com/help/optim/ug/passing-extra-parameters.html

[Charles]

Thank you very much for this.

A quick question on Syntax - what does the @ signify? Is this a pointer of sorts?

[Matt J]

@ has a number of different meanings in MATLAB. In this case, it is used to define an Anonymous Function, which hopefully you've read about by now

http://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html?searchHighlight=Anonymous+function

[Charles]

I have indeed, I was just wondering. Thanks again